无法创建文件 - ContextWrapper.getDir() 中出现 NullPointer 异常

Question

我已经尝试了一段时间了，想弄清楚如何在 Android 设备甚至是 SD 卡上保存数据以供应用程序读取。到目前为止还没有运气。我使用 Android SAX 解析器从服务器文件中读取 xml，甚至从 res/raw 文件夹中读取 xml。然而，我的麻烦在于编写 xml 文件本身。这只是一个测试程序，我目前的问题是，在尝试创建文件时，如 26 ( File dataDir = getDir("DataDir",0); ) 会生成错误。下面是代码，后面是 logcat。任何和所有的帮助，即使只是关于文件写入的一般帮助，都将不胜感激。谢谢。

MainActivity.java

package sample.matt.filemanip;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.ArrayList;
import org.xmlpull.v1.XmlSerializer;
import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.util.Xml;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends Activity {

    TextView tv;
    File dataDir = getDir("DataDir",0);
    final String fileLoc = dataDir.getPath()+"/stringfile.xml";
    private ArrayList<String> strings = new ArrayList<String>();

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        for(int r = 0; r<10; r++)
            strings.add("iteration "+r);

        tv = (TextView)findViewById(R.id.text_view);
        writeXmlToFile();

        loadFeed(ParserType.ANDROID_SAX);

    }

    private void loadFeed(ParserType type){
        try {
            Log.i("AndroidNews", "ParserType="+type.name());
            FeedParser parser = FeedParserFactory.getParser(fileLoc);
            strings = parser.parse();
            tv.setText(strings.get(0));
        } catch (Throwable t){
            Log.e("AndroidNews",t.getMessage(),t);
            Toast.makeText(MainActivity.this, "An error occured while reading quotes.", Toast.LENGTH_SHORT).show();
        }
    }

    private void writeXmlToFile(){

        File newxmlfile = new File(getCacheDir()+"/stringfile.xml");
        try{
                newxmlfile.createNewFile();
        }catch(IOException e){
                Log.e("IOException", "exception in createNewFile() method");
        }
        FileOutputStream fileos = null;        
        try{
                fileos = new FileOutputStream(newxmlfile);
        }catch(FileNotFoundException e){
                Log.e("FileNotFoundException", "can't create FileOutputStream");
        }
        XmlSerializer serializer = Xml.newSerializer();
        try {
                        serializer.setOutput(fileos, "UTF-8");
                        serializer.startDocument("", Boolean.valueOf(true));
                        //serializer.setFeature("http://xmlpull.org/v1/doc/features.html#indent-output", true);
                        serializer.startTag("", "root");
                        for (String str: strings){
                            serializer.startTag("", "string");
                                serializer.startTag(null, "content");
                                serializer.text(str);
                                serializer.endTag("", "content");
                            serializer.endTag(null, "string");
                        }
                        serializer.endTag(null, "root");
                        serializer.endDocument();
                        serializer.flush();
                        fileos.close();

                        tv.setText("file has been created");
                } catch (Exception e) {
                        Log.e("Exception","error occurred while creating xml file");
                }
    }
}

Logcat

07-13 12:03:02.499: ERROR/AndroidRuntime(7531): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{sample.matt.filemanip/sample.matt.filemanip.MainActivity}: java.lang.NullPointerException
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2585)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at android.app.ActivityThread.access$2300(ActivityThread.java:125)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at android.os.Handler.dispatchMessage(Handler.java:99)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at android.os.Looper.loop(Looper.java:123)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at android.app.ActivityThread.main(ActivityThread.java:4627)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at java.lang.reflect.Method.invokeNative(Native Method)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at java.lang.reflect.Method.invoke(Method.java:521)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at dalvik.system.NativeStart.main(Native Method)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531): Caused by: java.lang.NullPointerException
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at android.content.ContextWrapper.getDir(ContextWrapper.java:198)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at sample.matt.filemanip.MainActivity.<init>(MainActivity.java:21)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at java.lang.Class.newInstanceImpl(Native Method)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at java.lang.Class.newInstance(Class.java:1429)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
07-13 12:03:02.499: ERROR/AndroidRuntime(7531):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2577)

感谢您的宝贵时间，如果有不清楚的地方，请询问！

编辑：清单：

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="sample.matt.filemanip"
      android:versionCode="1"
      android:versionName="1.0">


    <application android:icon="@drawable/icon" android:label="@string/app_name">
        <activity android:name=".MainActivity"
                  android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

    </application>
</manifest>

编辑：新的 logcat 错误：

07-13 12:44:50.334: ERROR/AndroidNews(7640): Caused by: java.net.MalformedURLException: Protocol not found: /data/data/sample.matt.filemanip/app_DataDir/stringfile.xml
07-13 12:44:50.334: ERROR/AndroidNews(7640):     at java.net.URL.<init>(URL.java:275)
07-13 12:44:50.334: ERROR/AndroidNews(7640):     at java.net.URL.<init>(URL.java:159)
07-13 12:44:50.334: ERROR/AndroidNews(7640):     at sample.matt.filemanip.BaseFeedParser.<init>(BaseFeedParser.java:17)

Answer 1

也许 getDir() 方法是在超类的 onCreate() 方法中初始化的？尝试在onCreate()中调用getDir()。

Answer 2

不要在方法之外调用该方法。在您所做的位置声明 File dataDir，并在 onCreate 内调用 dataDir = getDir("DataDir",0);。下一个声明也是如此。

// Snip
File dataDir;
final String fileLoc;
private ArrayList<String> strings = new ArrayList<String>();

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    dataDir = getDir("DataDir",0);
    fileLoc = dataDir.getPath()+"/stringfile.xml";

// snap